1. Field of the Invention
This invention is concerned with a process for producing heat-curable, modified linear polyester resins. In particular, it is concerned with fluid coating compositions that are water-reducible. In another aspect, this invention is concerned with water-reducible fluid coating compositions suitable for coil-coating, and with the process in which such compositions are used for coating coils of metal.
2. Prior Art
Protective and decorative coatings are commonly applied to industrial products, such as appliances, automobiles, furniture, and sheet metal products. In most of these cases, the coating is applied as a fluid coating composition containing an organic solvent which is evaporated and discharged to the atmosphere.
The term "coating" as used herein is intended to mean the deposit produced in the coating process, and includes: the wet coating as initially deposited, including solvent; the dried coating substantially free of solvent; and the cured coating. The context will usually make clear whether wet coating, dried coating or cured coating is intended when the term "coating" is used without qualification. The term "fluid coating composition" is to be understood to mean a fluid composition useful in a coating process. The composition may require the addition of auxiliary materials prior to application. These auxiliary materials may include amounts of one or more of an organic solvent, water, a cross-linking agent, and a catalyst, said amounts being dictated by the requirements of the process.
Since a large portion of industrial finishes are heat-cured, evaporation often takes place inside of baking ovens, involving heating elements. With few exceptions, the organic solvents used are flammable, and their use requires that considerable precaution be taken to avoid fire or explosion. Many of the solvents are toxic at least to some degree. Because of the very large volume of industrial coatings that are used, discharge of the solvent vapors into the atmosphere represents an undesirable situation from an ecological point of view. Thus, industry has been seeking new types of coating systems which eliminate the use of solvent, or which use water as part or all of the solvent system. Example 7, U.S. Pat. No. 3,491,066, issued Jan. 20, 1970, illustrates one such water-reducible system.
It has become common practice in the fabrication of thin sheet metal products such as rain gutters, down spouts, and siding for houses, to apply the coating to the metal prior to forming the final product. In this practice, a coil of the sheet metal is fed, as more fully described hereinbelow, through a coating machine where the fluid coating composition is applied, the solvent evaporated, and the dried coating cured. The metal with the cured coating is picked up by a take-up roller where it is again formed into a coil before fabrication. This process will be referred to herein as "coil-coating." Formulation of coatings suitable for coil-coating must satisfy some stringent requirements. Generally, it is required that the coating cure fully in less than about 2 minutes, for example 40 seconds, compared with about 15 minutes for industrial coatings for refrigerators, for example. Thus, they must be fast curing. Also, they must be very adherent and flexible, so that the bond between the coating and the metal is maintained during the subsequent forming operation, which often involves making sharp bends, as in the manufacture of rain gutters.
The common currently used coil-coatings are formulated with linear polyester resins prepared with excess glycol. These hydroxyl-terminated resins are not water-reducible. Preparation of such resins with excess dibasic acid instead of excess glycol results in carboxyl-terminated polyester resins, which can be made water-reducible by the addition of amines, as is known in the art. However, such materials have not proved suitable for use in coil-coating, for example, because the reaction with the commonly added cross-linking agent, hexamethoxymethylmelamine, is not adequately fast.